Cold-rolled steel sheet and galvanized steel sheet, which are excellent in formability, panel shapeability, and dent-resistance, and method of manufacturing the same

ABSTRACT

Disclosed is a cold-rolled steel sheet excellent in formability, panel shapeability and dent-resistance, comprising 0.005 to 0.015% by weight of C, 0.01 to 0.2% by weight of Si, 0.2 to 1.5% by weight of Mn, 0.01 to 0.07% by weight of P, 0.006 to 0.015% by weight of S, 0.01 to 0.08% by weight of sol. Al, not higher than 0.004% by weight of N (N≦0.004%), not higher than 0.003% by weight of O (O≦0.003%), 0.04 to 0.23% by weight of Nb, 1.0≦(Nb %×12)/(C %×93)≦3.0, and a balance of Fe and unavoidable impurities, said cold-rolled steel sheet meeting the relationship given below: 
     
       
         exp(ε)×(5.29×exp(ε)−4.19)≦σ/σ 0.2 ≦exp(ε)×(5.64×exp(ε)−4.49) 
       
     
     where 0.002&lt;ε≧0.096, ε represents a true strain, σ 0.2  represents a 0.2% proof stress, and σ represents a true stress relative to ε.

TECHNICAL FIELD

The present invention relates to a cold-rolled steel sheet and a galvanized steel sheet, which are excellent in formability, panel shapeability, and dent-resistance required for an outer panel of a motor car, and a method of manufacturing the same.

BACKGROUND ART

An excellent formability, a satisfactory shape after a panel formation and a high dent-resistance (resistance to local depression) are required for a steel sheet for an outer panel of a motor car. The panel formability is evaluated by indexes such as yield strength, elongation, and an n-value (work-hardening index) of the steel sheet. Also, the panel shapeability and the dent-resistance are evaluated in many cases by yield strength and the yield strength after the working and the coating-baking treatment. If the yield strength of the steel sheet is weakened, the press formability can be improved. However, the dent-resistance after the panel formation is rendered unsatisfactory. On the other hand, if the yield strength of the steel sheet is increased, the dent-resistance is improved. However, problems are generated in terms of the press formability such as occurrence of wrinkles or cracks. Such being the situation, vigorous researches are being made in an attempt to obtain a steel sheet having a low yield point in the press forming and a high yield strength after the forming and baking as an outer panel for a motor car. As a cold-rolled steel sheet meeting these two contradictory requirements in terms of the yield strength, a bake-hardenable steel sheet, hereinafter referred to as a “BH steel sheet”, utilizing a strain aging phenomenon of the carbon atoms within the steel has been developed.

Particularly, known is a method of manufacturing a BH steel sheet having an excellent deep drawability, which is a cold-rolled steel sheet prepared by adding elements capable of forming carbonitrides such as Nb and Ti to a steel having a very low carbon content of about 50 ppm, the addition amount of such an element being not larger than 1 in terms of the atomic ratio of carbon. For example, Japanese Patent Publication (Kokoku) No. 60-46166 teaches that a Nb or Ti added low-carbon steel is annealed at a high temperature close to 900° C. for manufacturing the particular BH steel sheet. Also, Japanese Patent Disclosure (Kokai) No. 61-276928 teaches that an extra low carbon BH steel sheet is manufactured by annealing under a temperature region of about 700 to 850° C.

The technology disclosed in JP '166 is certainly advantageous in that the BH properties and an r-value can be improved. However, since the annealing is performed at a high temperature, the rough surface derived from enlargement of the ferrite grains is worried about. In addition, since the steel sheet itself is softened, the yield strength after the press forming and the baking steps is not acceptably high, though high BH properties may be obtained. On the other hand, in the technology disclosed in JP '928, the annealing temperature is relatively low, compared with that employed in JP '166, and, thus, is desirable in the required surface properties and the yield strength. However, it is substantially impossible to improve as desired the BH properties and the r-value. It should also be noted that these prior arts are mainly intended to improve the BH properties of a steel sheet in order to allow the steel sheet to exhibit an improved dent-resistance. Therefore, deterioration in the resistance to natural aging, i.e., occurrence of stretcher strain in the press forming, which is derived from generation of a yield point elongation during storage under room temperature, is worried about. Under the circumstances, the BH amount is suppressed at 60 MPa or less in view of the practical use of the steel sheet.

As described above, the cold-rolled sheet manufactured by the conventional method is not sufficiently satisfactory in the surface properties, the resistance to natural aging, and the dent-resistance, which are required for the steel sheet used for an outer panel of a motor car.

An object of the present invention is to provide a cold-rolled steel sheet and a galvanized steel sheet, which are satisfactory in any of the surface properties, the resistance to natural aging, and the dent-resistance, which are required for the steel sheet used for an outer panel of a motor car, and a method of manufacturing the same.

DISCLOSURE OF INVENTION

(1) The present invention provides a cold-rolled steel sheet excellent in formability, panel shapeability and dent-resistance, comprising 0.005 to 0.015% by weight of C, 0.01 to 0.2% by weight of Si, 0.2 to 1.5% by weight of Mn, 0.01 to 0.07% by weight of P, 0.006 to 0.015% by weight of S, 0.01 to 0.08% by weight of sol. Al, not higher than 0.004% by weight of N, not higher than 0.003% by weight of O, 0.04 to 0.23% by weight of Nb, the amounts of Nb and C meeting the relationship given in formula (1), and a balance of Fe and unavoidable impurities, the cold-rolled steel sheet meeting the relationship given in formula (2):

1.0≦(Nb %×12)/(C %×93)≦3.0  (1)

 exp(ε)×(5.29×exp(ε)−4.19)≦σ/σ_(0.2)≦exp(ε)×(5.64×exp(ε)−4.49)  (2)

where 0.002<ε≦0.096, ε represents a true strain, σ_(0.2) represents a 0.2% proof stress, and a represents a true stress relative to σ.

(2) The present invention provides the cold-rolled steel sheet excellent in formability, panel shapeability and dent-resistance defined in item (1) above, further comprising 0.0001 to 0.002% by weight of B.

(3) The present invention provides a galvanized steel sheet excellent in formability, panel shapeability and dent-resistance, which is obtained by applying a galvanizing to the cold-rolled steel sheet defined in item (1) or item (2) above.

(4) The present invention provides a method of manufacturing a cold-rolled steel sheet excellent in formability, panel shapeability and dent-resistance defined in item (1) or item (2) above, comprising the steps of:

preparing a molten steel and continuously casting the steel;

applying a hot-rolling process such that a finish rolling is performed at (Ar₃−100)° C. or more and the rolled steel sheet is coiled at 500 to 700° C.; and

continuously applying a cold-rolling process and an annealing process to the hot-rolled steel sheet.

(5) The present invention provides a method of manufacturing a galvanized steel sheet, the steel sheet being excellent in formability, panel shapeability and dent-resistance, defined in item (3) above, comprising the steps of:

preparing a molten steel and continuously casting the steel;

applying a hot-rolling process such that a finish rolling is performed at (Ar₃−100)° C. or more and the rolled steel sheet is coiled at 500 to 700° C.; and

continuously applying a cold-rolling process and a galvanizing process to the hot-rolled steel sheet.

(6) The present invention provides a cold-rolled steel sheet excellent in the surface shape of a panel and dent-resistance, comprising 0.004 to 0.015% by weight of C, 0.01 to 0.2% by weight of Si, 0.1 to 1.5% by weight of Mn, 0.01 to 0.07% by weight of P, 0.005 to 0.015% by weight of S, 0.01 to 0.08% by weight of sol. Al, not higher than 0.005% by weight of N, and at least one kind of the element selected from the group consisting of 0.02 to 0.12% by weight of Nb and 0.03 to 0.1% by weight of Ti, the amount of C, Nb, Ti, N and S meeting the relationship given in formula (1), and a balance of Fe and unavoidable impurities, the cold-rolled steel sheet meeting the relationship given in formula (2):

 −0.001≦C %−(12/93)Nb %−(12/48)Ti*≦0.001  (1)

where Ti*=Ti %−(48/14)N %−(48/32)S %, when Ti* is not larger than 0, Ti* is regarded as 0.

exp(ε)×(5.29×exp(ε)−4.19)≦σ/σ_(0.2)≦exp(ε)×(5.64×exp(ε)−4.49)  (2)

where 0.002 <ε≦0.096, ε represents a true strain, σ0.2 represents a 0.2% proof stress, and σ represents a true stress relative to ε.

(7) The present invention provides a cold-rolled steel sheet excellent in the surface shape of a panel and dent-resistance defined in item (6) above, further comprising 0.0001 to 0.002% by weight of B.

(8) The present invention provides a galvanized steel sheet, the steel sheet being excellent in the surface shape of a panel and dent-resistance and prepared by applying a galvanizing to the cold-rolled steel sheet defined in item (6) or item (7) above.

(9) The present invention provides a method of manufacturing a cold-rolled steel sheet excellent in the surface shape of a panel and dent-resistance and defined in item (6) or item (7) above, comprising the steps of:

applying a hot-rolling process after preparation of a molten steel and continuous casting of the steel such that a finish rolling is performed at (Ar₃−100)° C. or more and the rolled steel sheet is coiled at 500 to 700° C.; and

continuously applying a cold-rolling process and an annealing process to the hot-rolled steel sheet.

(10) The present invention provides a method of manufacturing a galvanized steel sheet, the steel sheet being excellent in the surface shape of a panel and dent-resistance and defined in item (8) above, comprising the steps of:

applying a hot-rolling process after preparation of an ingot steel and continuous casting of the ingot steel such that a finish rolling is performed at (Ar₃−100)° C. or more and the rolled steel sheet is coiled up at 500 to 700° C.; and

continuously applying a cold-rolling treatment and a galvanizing treatment to the hot-rolled steel band.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B show the relationships between the elongation El and (Nb×12)/(C×93) and between the r-value and (Nb×12)/(C×93) according to a first embodiment of the present invention;

FIG. 2 shows a method of evaluating the dent-resistance and the shapeability according to the first embodiment of the present invention;

FIG. 3 is a graph showing how P0.1 (dent-resistance load of a panel imparted with strains of 2%, 4% and 8%) and δ (spring back amount of 2% panel) are affected by σ/σ_(0.2) exp(ε), and components of the steel composition according to the first embodiment of the present invention;

FIG. 4 is a graph showing how P0.1 (dent-resistance load of a panel imparted with strains of 2%, 4% and 8%) and δ (spring back amount of 2% panel) are affected by σ/σ_(0.2), exp(ε), and components of the steel composition according to the first embodiment of the present invention;

FIG. 5 is a graph showing how P0.1 (dent-resistance load of a panel imparted with strains of 2%, 4% and 8%) and δ (spring back amount of 2% panel) are affected by σ/σ_(0.2), exp(ε), and components of the steel composition according to the first embodiment of the present invention;

FIG. 6 is a graph showing how the finishing temperature and the coiling temperature have an influence on P0.1 (dent-resistance load of a panel imparted with strains of 2%), δ, and Wca (Arithmetic Average Waviness Height) according to the first embodiment of the present invention;

FIG. 7 shows how an experiment for evaluating the dent-resistance and the shapeability is conducted according to a second embodiment of the present invention;

FIG. 8 is a graph showing how P0.1 (dent-resistance load of a panel imparted with strains of 2%, 4% and 8%) and δ (spring back amount of 2% panel) are affected by σ/σ_(0.2), exp(ε), and components of the steel composition according to the second embodiment of the present invention;

FIG. 9 is a graph showing how P0.1 (dent-resistance load of a panel imparted with strains of 2%, 4% and 8%) and δ (spring back amount of 2% panel) are affected by σ/σ_(0.2), exp(ε), and components of the steel composition according to the second embodiment of the present invention;

FIG. 10 is a graph showing how P0.1 (dent-resistance load of a panel imparted with strains of 2%, 4% and 8%) and δ (spring back amount of 2% panel) are affected by σ/σ_(0.2), exp(ε), and components of the steel composition according to the second embodiment of the present invention;

FIG. 11 is a graph showing how the finishing temperature and the coiling temperature have an influence on P0.1 (dent-resistance load of a panel imparted with strains of 2%), δ, and Wca (Arithmetic Average Waviness Height) according to the second embodiment of the present invention; and

FIG. 12 is a graph showing the relationship between the storage time and ΔYPel (recovery amount of YPel in the case of storage at 25° C. after the temper rolling) in Example 3 of the second embodiment of the present invention.

BEST MODE OF CARRYING OUT THE INVENTION

The present inventors have conducted an extensive research in an attempt to obtain a cold-rolled steel sheet and a galvanized steel sheet, which are excellent in the surface properties, the resistance to natural aging and the dent-resistance required for the steel used for an outer panel of a motor car, and a method of manufacturing the same.

As a result, it has been found that the dent-resistance of a panel can be improved by an alloy design with an emphasis placed on the work-hardening behavior in a low strain region in the panel forming step, unlike the prior art in which the dent-resistance required for ah outer panel of a motor car is improved by increasing the BH value. It has also been found that good surface properties and resistance to natural aging can be imparted to the steel sheet by positively suppressing the BH value. These findings have enabled the present inventors to develop a technology for stably manufacturing a cold-rolled steel sheet and a galvanized steel sheet, being excellent in the panel surface shapeability and the dent-resistance and exhibiting such a high tensile strength as at least 340 MPa.

Some embodiments of the present invention will now be described.

First Embodiment

Described in the following are the reasons for using the additives, the reasons for limiting the amounts of the additives, the reasons for limiting the tensile characteristics, and the reasons for limiting the manufacturing conditions according to the first embodiment of the present invention. In the following description, “%” represents “% by weight”.

(1) Amounts of Additives

C: 0.005 to 0.015%

A carbide formed together with Nb affects the work-hardening in a low strain region in panel forming step and contributes to an improvement of the dent-resistance. The particular effect cannot be obtained, if the C amount is less than 0.005%. Also, if the C amount exceeds 0.015%, the dent-resistance of the panel is certainly improved. However, the shape of the panel is impaired. It follows that the C amount should fall within a range of between 0.005 and 0.015%.

Si: 0.01 to 0.2%

Silicon is effective for strengthening the steel. However, if the Si amount is smaller than 0.01%, it is impossible to obtain a capability of the solid solution strengthening. On the other hand, if the Si amount is larger than 0.2%, the surface properties of the steel sheet are impaired. In addition, striped surface defects are generated after galvanizing. Therefore, the Si amount should fall within a range of between 0.01 and 0.2%.

Mn: 0.2 to 1.5%

Manganese serves to precipitate sulfide and to suppress deterioration of the hot ductility. Also, Mn is effective for strengthening the steel. If the Mn amount is less than 0.2%, hot brittleness of the steel sheet is brought about, leading to a low yield. In addition, a high mechanical strength characterizing the steel sheet of the present invention cannot be obtained. Further, Mn, which relates to an improvement in the workability of the steel sheet, is necessary for controlling the morphology of the MnS in the hot rolling step. It should be noted that fine MnS particles are formed by the process of resolution and re-precipitation in the hot rolling step. These MnS particles impair the grain growth of the steel. However, if Mn is added in an amount not smaller than 0.2%, it is possible to eliminate the above-noted adverse effect produced by the presence of the MnS particles. In order to control effectively the morphology of the MnS particles in the hot rolling step, it is more desirable to add Mn in an amount of at least 0.45%. However, if the Mn amount exceeds 1.5%, the steel sheet is hardened and the panel shapeability of the steel sheet are deteriorated. It follows that Mn amount should fall within a range of between 0.2% and 1.5%.

P: 0.01 to 0.07%

Phosphorus is most effective for the solid solution strengthening of steel. If the P amount is smaller than 0.01%, however, P fails to exhibit a sufficient strengthening capability. On the other hand, if the P amount exceeds 0.07%, the ductility of the steel sheet is deteriorated. Also, a defective coating is brought about in the step of the alloying treatment during the continuous galvanizing process. It follows that the P amount should fall within a range of between 0.01 and 0.07%.

S: 0.006 to 0.015%

Sulfur, if added in an amount exceeding 0.015%, brings about hot brittleness of the steel. If the S amount is smaller than 0.006%, however, the peeling capability of the scale is impaired in the hot rolling step, and surface defects tend to be generated markedly. It follows that the S amount should fall within a range of between 0.006 and 0.015%.

Sol. Al: 0.01 to 0.08%

Aluminum serves to deoxidize the steel and fix N as nitride. If the Al amount is smaller than 0.01%, however, the deoxidation and the fixation of N cannot be achieved sufficiently. On the other hand, if the Al amount is larger than 0.08%, the surface properties of the steel sheet are deteriorated. Therefore, the Al amount should fall within a range of between 0.01 and 0.08%.

N≦0.004%

Nitrogen is fixed in the form of AlN. If the N amount exceeds 0.004%, however, it is impossible to obtain a desired formability of the steel sheet. Naturally, the N amount should not exceed 0.004%.

O≦0.003%

Oxygen forms inclusions involving oxides so as to adversely affect the grain growth of the steel. If the O amount exceeds 0.003%, the grain growth is impaired in the annealing step, resulting in failure to obtain satisfactory formability and panel shapeability. Naturally, the O amount should not exceed 0.003%. In order to suppress the O amount at 0.003% or less in the steel of the composition specified in the present invention, it is necessary to employ optimum manufacturing conditions. For example, the sol. Al should be controlled at a suitable level, and O should be controlled up in the process steps after the secondary refining process.

Nb: 0.04 to 0.23%

Niobium is bonded to C to form fine carbide particles. These fine carbide particles affect the work-hardening behavior in the panel forming step so as to contribute to an improvement in the dent-resistance of the panel. If the Nb amount is smaller than 0.04%, however, it is impossible to obtain the particular effect. On the other hand, if the Nb amount exceeds 0.23%, the panel shapeability such as the spring back and the surface deflection is deteriorated, though the dent-resistance is certainly improved. Naturally, the Nb amount should fall within a range of between 0.04 and 0.23%.

(Nb×12)/(C×93): 1.0 to 3.0

In the present invention, it is absolutely necessary to control (Nb×12)/(C×93) in order to improve the formability of the steel sheet. If the value of (Nb×12)/(C×93) is less than 1.0, C cannot be fixed sufficiently, resulting in failure to obtain a high r-value and a high ductility aimed at in the present invention. If the value exceeds 3.0, however, the amount of Nb forming a solid solution is rendered excessively high, leading to a low ductility. In this case, it is impossible to obtain a formability aimed at in the present invention. It follows that the value of (Nb×12)/(C×93) should fall within a range of between 1.0 and 3.0. FIGS. 1A and 1B show the relationships between the elongation El and (Nb×12)/(C×93) and between the r-value and (Nb×12)/(C×93).

In order to improve the dent-resistance as desired, it is desirable to add B in an amount given below in addition to the additives described above.

B: 0.0001 to 0.002%

If B is added, the grain boundary is strengthened so as to improve the resistance to the secondary working brittleness. Also, the ferrite grains are diminished so as to ensure an absolute value of the yield strength and, thus, to improve the dent-resistance. However, these effects cannot be obtained if the B amount is smaller than 0.0001%. On the other hand, if the B amount exceeds 0.002%, the yield point is increased and, thus, the panel shapeability is impaired. It follows that the B amount should fall within a range of between 0.0001 and 0.002%.

(2) Tensile Characteristics

exp(ε)×(5.29×exp(ε)−4.19)≦σ/σ_(0.2)≦exp(ε)×(5.64×exp(ε)−4.49), where 0.002≦ε0.096, ε represents a true strain, σ_(0.2) represents a 0.2% proof stress, and a represents a true stress relative to ε.

In the steel sheet of the present invention comprising the additives described in item (1) above, Fe and unavoidable impurities, a ratio of flow stress a obtained by a tensile test under the condition that a true strain ε is larger than 0.002 and not larger than 0.096, i.e., 0.002<ε≦0.096, to a 0.2% proof stress σ_(0.2) i.e., σ/σ_(0.2), should fall within a range of between exp(ε)×(5.29×exp(ε)−4.19) and exp(ε)×(5.64×exp(ε)−4.49).

If the ratio σ/σ_(0.2) is lower than the lower limit noted above, the dent-resistance load under the conditions of 2%P0.1, 4%P0.1, 8%P0.1 is as high as 160 to 190N as shown in FIGS. 3 to 5. For measuring the dent-resistance load, a steel sheet is formed to a model panel shown in FIG. 2 with strain of 2%, 4% or 8% imparted to the steel sheet, followed by applying a heat treatment at 170° C. for 20 minutes. Then, measured is a load required for imparting a residual displacement of 0.1 mm to the model panel. However, the spring back δ (measured for a panel having a strain of 2%) is as large as 7 to 10% so as to impair the panel shapeability, if the ratio σ/σ_(0.2) is lower than the lower limit noted above. On the other hand, if the ratio σ/σ_(0.2) is higher than the upper limit noted above, the spring back δ is as small as 2 to 5% to improve the panel shapeability. However, the dent-resistance is as low as 140 to 175N. In other words, the dent-resistance cannot be improved. Under the circumstances, the ratio σ/σ_(0.2) should fall within a range of between exp(ε)×(5.29×exp(ε)−4.19) and exp(ε)×(5.64×exp(ε)−4.49).

A cold-rolled steel sheet and a galvanized steel sheet excellent in the panel surface properties and the dent-resistance required for the steel used for an outer panel of a motor car can be obtained by controlling the additive components as described in item (1) above and the tensile characteristics as described in item (2) above.

The steel sheet exhibiting the particular properties can be manufactured as follows.

(3) Steel Sheet Manufacturing Process

In the first step, steel of the composition given in item (1) above is melted. A converter method is generally employed for melting the steel composition, or an electric furnace method can also be employed. After the molten steel is continuously cast to obtain a slab, the slab is heated immediately after the casting, or after the slab is once cooled, for applying a hot rolling. The hot rolling is performed under the conditions that the finishing temperature is set at temperature not less than (Ar₃−100)° C. and that the coiling temperature is set at 500° C. to 700° C. If the finishing temperature is lower than (Ar₃−100)° C., 2%P0.1, i.e., the dent-resistance load of the panel imparted with 2% of strain) is as low as 140 to 150N, as shown in FIG. 6. In other words, the dent-resistance of the panel cannot be improved. Also, where the coiling temperature is lower than 500° C., the value of 2%P0.1 is high, i.e., 155 to 165N. However, the value of δ, i.e., the spring back amount of the panel imparted with 2% of strain, is as large as 8% to 10%, leading to a poor shapeability. On the other hand, where the coiling temperature exceeds 700° C., the value of Wca (i.e., Arithmetic Average Waviness Height; measuring length of 25 mm; average of the values measured at 10 optional points around the apex of the panel) is large, which falls within a range of between a value exceeding 0.4 μm and 0.6 μm, leading to a poor panel shapeability. It follows that the finishing temperature should be not lower than (Ar₃−100)° C. and that the coiling temperature should fall within a range of between 500° C. and 700° C.

In the next step, the hot-rolled steel band is subjected to pickling, cold-rolling and, then, a continuous annealing. Alternatively, galvanizing is applied after the continuous annealing. The cold-rolling reduction should desirably be at least 70% in order to improve the deep drawability (r-value) of the steel sheet. The annealing should desirably be carried out within a recrystallization temperature region of the ferrite phase. Further, the coating employed in the present invention is not limited to continuous galvanizing. Specifically, even if a surface treatment such as coating with zinc phosphate or an electrolytic galvanizing is applied to the steel sheet obtained by the continuous annealing, no problem is brought about in the characteristics of the resultant steel sheet.

Second Embodiment

Described in the following are the reasons for using the additives, the reasons for limiting the amounts of the additives, the reasons for limiting the tensile characteristics, and the reasons for limiting the manufacturing conditions according to the second embodiment of the present invention. In the following description, “%” represents “% by weight”.

(1) Amounts of Additives

C: 0.004 to 0.015%

A carbide formed together with Nb or Ti affects the work-hardening in a low strain region in the panel forming step and contributes to an improvement of the dent-resistance. The particular effect cannot be obtained, if the C amount is less than 0.004%. Also, if the C amount exceeds 0.015%, the dent-resistance of the panel is certainly improved. However, the shape of the panel is impaired. It follows that the C amount should fall within a range of between 0.004 and 0.015%.

Si: 0.01 to 0.2%

Silicon is effective for strengthening the steel. However, if the Si amount is smaller than 0.01%, it is impossible to obtain a capability of strengthening. On the other hand, if the Si amount is larger than 0.2%, the surface properties of the steel sheet are impaired. In addition, striped surface defects are generated after galvanizing. Therefore, the Si amount should fall within a range of between 0.01 and 0.2%.

Mn: 0.1 to 1.5%

Manganese serves to precipitate sulfide and to suppress deterioration of the hot ductility. Also, Mn is effective for strengthening the steel. If the Mn amount is less than 0.1%, hot brittleness of the steel sheet is brought about. However, if the Mn amount exceeds 1.5%, the steel sheet is hardened and the panel shapeability of the steel sheet is deteriorated. It follows that Mn amount should fall within a range of between 0.1% and 1.5%.

P: 0.01 to 0.07%

Phosphorus is most effective for strengthening the steel. If the P amount is smaller than 0.01%, however, P fails to exhibit a sufficient strengthening capability. On the other hand, if the P amount exceeds 0.07%, the ductility of the steel sheet is deteriorated. Also, a defective coating is brought about in the step of the alloying treatment during the process of the continuous galvanizing. It follows that the P amount should fall within a range of between 0.01 and 0.07%.

S: 0.005 to 0.015%

Sulfur, if added in an amount exceeding 0.015%, brings about hot brittleness of the steel. However, the S amount smaller than 0.005% is undesirable in terms of the manufacturing cost of the desired steel sheet because a desulfurization treatment and a degassing treatment of the molten steel are required. It follows that the S amount should fall within a range of between 0.005 and 0.015%.

Sol. Al: 0.01 to 0.08%

Aluminum serves to deoxidize the steel. If the Al amount is smaller than 0.01%, however, the deoxidation cannot be achieved sufficiently. On the other hand, if the Al amount is larger than 0.08%, the surface properties of the steel sheet are deteriorated. Therefore, the Al amount should fall within a range of between 0.01 and 0.08%.

N≦0.005%

Nitrogen is fixed in the form of TiN. If the N amount exceeds 0.005%, however, the resistance to natural aging is deteriorated. Naturally, the N amount should not exceed 0.005%.

Nb: 0.02 to 0.12%

Niobium is bonded to C to form fine carbide particles. These fine carbide particles affect the work-hardening behavior in the panel forming step so as to contribute to an improvement in the dent-resistance of the panel. If the Nb amount is smaller than 0.02%, however, it is impossible to obtain the particular effect. On the other hand, if the Nb amount exceeds 0.12%, the panel shapeability such as the spring back and the surface deflection is deteriorated, though the dent-resistance is certainly improved. Naturally, the Nb amount should fall within a range of between 0.02 and 0.12%.

Ti: 0.03 to 0.1%

Like Nb, Ti forms fine carbide particles. These fine carbide particles greatly contribute to an improvement in.the dent-resistance of the panel. If the Ti amount is smaller than 0.03%, however, it is impossible to obtain the particular effect. On the other hand, if the Ti amount exceeds 0.1%, the panel shapeability is deteriorated. Also, the surface of the galvanized steel sheet is impaired. Naturally, the Ti amount should fall within a range of between 0.03 and 0.1%.

−0.001≦C %−(12/93)Nb %−(12/48)Ti*≦0.001,

where Ti*=Ti %−(48/14)N %−(48/32)S %, when Ti* is not larger than 0, Ti* is regarded as 0.

In the present invention, the value of C %−(12/93)Nb %−(12/48)Ti* (where Ti*=Ti %−(48/14)N %−(48/32)S %, when Ti* is not larger than 0, Ti* is regarded as 0, which is defined by C, Nb and Ti) should be at least −0.001% and should not exceed 0.001%. If the value exceeds 0.001%, the resistance to natural aging is deteriorated. Also, if the value is smaller than −0.001%, Nb forming a solid solution or Ti forming a solid solution is increased so as to impair the surface properties of the steel sheet and increase the yield point, leading to deterioration of the panel shapeability.

In the present invention, it is also possible to add B in an amount given below in addition to the additives described above in order to improve the resistance to the secondary working brittleness and the dent-resistance.

B: 0.0001 to 0.002%

If B is added, the grain boundary is strengthened so as to improve the resistance to the secondary working brittleness. Also, the ferrite grains are diminished so as to ensure an absolute value of the yield strength and, thus, to improve the dent-resistance. However, these effects cannot be obtained if the B amount is smaller than 0.0001%. On the other hand, if the B amount exceeds 0.002%, the yield point is increased and, thus, the panel shapeability is impaired. It follows that the B amount should fall within a range of between 0.0001 and 0.002%.

(2) Tensile Characteristics

exp(ε)×(5.29×exp(ε)−4.19)≦σ/σ_(0.2)≦exp(ε)×(5.64×exp(ε)−4.49), where 0.002<ε≦0.096, ε represents a true strain, σ_(0.2) represents a 0.2% proof stress, and σ represents a true stress relative to ε.

In the steel sheet of the present invention comprising the additives described in item (1) above, Fe and unavoidable impurities, a ratio of flow stress a obtained by a tensile test under the condition that a true strain ε is larger than 0.002 and not larger than 0.096, i.e., 0.002<ε≦0.096, to a 0.2% proof stress σ_(0.2), i.e., σ/σ_(0.2), should fall within a range of between exp(ε)×(5.29×exp(ε)−4.19) and exp(ε)×(5.64×exp(ε)−4.49).

If the ratio σ/σ_(0.2) is lower than the lower limit noted above, the dent-resistance load under the conditions of 2%P0.1, 4%P0.1, 8%P0.1 is as high as 160 to 210N as shown in FIGS. 8 to 10. For measuring the dent-resistance load, a steel sheet is shaped into a model panel shown in FIG. 1 with strain of 2%, 4% or 8% imparted to the steel sheet, followed by applying a heat treatment at 170° C. for 20 minutes. Then, measured is a load required for imparting a residual displacement of 0.1 mm to the model panel. However, the spring back δ (measured for a panel having a strain of 2%) is as large as 7 to 11% so as to impair the panel shapeability, if the ratio σ/σ_(0.2) is lower than the lower limit noted above. On the other hand, if the ratio σ/σ_(0.2) is higher than the upper limit noted above, the spring back δ is as small as 1 to 5%. However, the dent-resistance is as low as 140 to 165N. In other words, the dent-resistance cannot be improved.

A cold-rolled steel sheet and a galvanizing steel sheet excellent in the panel surface properties, the resistance to natural aging and the dent-resistance required for the steel used for an outer panel of a motor car can be obtained by controlling the additive components as described in item (1) above and the tensile characteristics as described in item (2) above.

The steel sheet exhibiting the particular properties can be manufactured as follows.

(3) Steel Sheet Manufacturing Process

In the first step, steel of the composition given in item (1) above is melted. A converter method is generally employed for melting the steel composition, or an electric furnace method can also be employed. After the molten steel is continuously cast to obtain a slab, the slab is heated to 1050° C. or higher immediately after the casting, or after the slab is once cooled, for applying a hot rolling. The hot rolling is performed under the conditions that the finishing temperature is set at temperature not less than (Ar₃−100)° C. and that the coiling temperature is set at 500° C. to 700° C. If the finishing temperature is lower than (Ar₃−100)° C., 2%P0.1, i.e., the dent-resistance load of the panel imparted with 2% of strain) is as low as 140 to 155N, as shown in FIG. 11. In other words, the dent-resistance of the panel cannot be improved. Also, where the coiling temperature is lower than 500° C. or higher than 700° C., the value of 2%P0.1 is high, i.e., 156 to 175N. However, the value of Wca, (i.e., Arithmetic Average Waviness Height;, measuring length of 25 mm; average of the values measured at 10 optional points around the apex of the panel) is large, which falls within a range of between a value exceeding 0.2 μm and 0.6 μm, leading to a poor panel shapeability.

In the next step, the hot-rolled steel band is subjected to a pickling, cold-rolling and, then, a continuous annealing. Alternatively, galvanizing is applied after the continuous annealing step. The cold-rolling reduction should desirably be at least 70% in order to improve the deep drawability of the steel sheet. The annealing should desirably be carried out within a recrystallization temperature region of the ferrite phase and not higher than 930° C. Further, the coating employed in the present invention is not limited to galvanizing. Specifically, even if a surface treatment such as coating with zinc phosphate or an electrolytic zinc coating is applied to the steel sheet obtained by the continuous annealing, no problem is brought about in the characteristics of the resultant steel sheet.

Some Examples of the present invention will now be described to demonstrate the prominent effects produced by the present invention.

EXAMPLES Example 1

Molten steel of the composition shown in Table 1 were prepared in a laboratory, followed by continuously casting the steel to prepare a slab having a thickness of 60 mm. Samples Nos. 1 to 7 shown in Table 1 represent the steel of the composition specified in the present invention, with samples Nos. 8 to 15 denoting the steel for Comparative Examples. The slab was treated by a blooming mill to reduce the thickness of the steel sheet to 30 mm, followed by heating the steel sheet at 1050° C. for 1.5 hours under the atmosphere for the hot rolling treatment (by roughing mill). After the rough rolling, a finish rolling was applied at 900° C., followed by applying a coiling simulation at 630° C. so as to obtain a hot rolled sheet having a thickness of 3 mm. Then, the hot rolled steel sheet was pickled, followed by applying a cold rolling to reduce the thickness of the steel sheet to 0.8 mm and subsequently applying a continuous annealing at 840° C. for 90 seconds. Alternatively, after the continuous annealing at 840° C. for 90 seconds, a galvanizing was applied at 460° C., followed by applying an alloying treatment at 530° C. Further, 1.0% of temper rolling was applied to the annealed steel sheet or the galvanized steel sheet so as to prepare samples for the experiments. These samples were used for the tensile test (test piece of JIS No. 5; tested in accordance with the method specified in JIS Z 2241) and for measuring the r-value, 2% BH amount (measured in accordance with the method specified in JIS G 3135), and ΔYPel (restoring amount of yield point elongation of the sample stored at 25° C. for 6 months after the temper rolling). Also, the sample was formed into the model panel shown in FIG. 2 (formed at three levels of the forming strain of 2, 4 and 8%). After a heat treatment was applied at 170° C. for 20 minutes, the dent-resistance of the panel and the shapeability of the panel were examined. The dent-resistance was evaluated under a load of P0.1, in which 0.1 mm of residual displacement was imparted to the panel (in the following description, expressions of 2%P0.1, 4%P0.1 and 8%P0.1 are used for denoting the panel imparted with strain of 2, 4 and 8%, respectively). On the other hand, the panel shapeability was evaluated by the spring back amount δ and Wca: Arithmetic Average Waviness Height (JIS B 0610). The spring back amount δ was defined by using a curvature radius R′ of the panel imparted with 2% of strain and a curvature radius R of the press mold, i.e., δ was defined by (R′/R—1)×100. Where δ was not larger than 6%, i.e., δ>6%, the evaluation was marked by ◯. Where δ was 7 to 10%, i.e., δ=7 to 10%, the evaluation was marked by Δ. Further, where δ was larger than 10%, i.e., δ>10%, the evaluation was marked by x. On the other hand, the surface waviness height each having a length of 25 mm were measured at optional 10 points in the vicinity of the apex of the panel, and the average measured value is denoted by Wca. Where Wca was not larger than 0.2 μm, i.e., Wca≦0.2 μm, the evaluation was marked by ◯. Where Wca was larger than 0.2 μm but not larger than 0.4 μm, i.e., 0.2 μm<Wca≦0.4 μm, the evaluation was marked by Δ. Further, where Wca was larger than 0.4 μm and not larger than 0.6 μm, i.e., 0.4 μm<Wca≦0.6 μm, the evaluation was marked by x.

Table 2 shows the results of measurements and evaluations. In samples Nos. 1 to 7 each having a composition falling within the range specified in the present invention, the value of the elongation El was as large as 41.6% to 45.0%. The average r-value, i.e., (r0+2r45+r90)/4, was as large as 1.80 to 2.20. The value of ΔYPel was 0% in any of the samples of the present invention. On the other hand, the spring back amount δ and the Waviness Height Wca were small, i.e., 3% to 5% and 0.09 μm to 0.17 μm, respectively, supporting a good panel shapeability. Further, the dent-resistance P0.1 of the panel imparted with strains of 2%, 4% and 8% was as high as 158N to 193N.

On the other hand, the steel samples Nos. 8 to 15, each having a composition failing to fall within the range specified in the present invention, did not satisfy simultaneously the formability, the shapeability, and the dent-resistance. Specifically, each of Comparative Samples Nos. 8 and 9 exhibited a 2% BH as high as 33 MPa to 42 MPa and a ΔYPel of 0.9% to 2.2%, indicating that these samples were not satisfactory in the resistance to natural aging. Also, the dent-resistance P0.1 under strains of 2% to 8% was found to be 165N to 193N, supporting a high dent-resistance. However, each of these Comparative samples was low in each of the elongation El and the r-value and large in each of the spring back amount δ and the value of Wca, supporting that these Comparative samples were not satisfactory in formability and shapeability. Comparative steel sample No. 10 was high in the elongation El and the r-value, and low in δ and Wca, supporting that this sample was satisfactory in each of formability and shapeability. However, the dent-resistance load P0.1 under strains of 2% to 8% was as low as 148 to 172N. Comparative steel sample No. 11 was high in σ_(0.2), which was 265 MPa to 270 MPa, supporting that this sample was satisfactory in dent-resistance. However, the steel sample was high in each of δ and Wca, supporting a poor panel shape. Further, this steel sample was low in the elongation El and the r-value. Each of Comparative steel samples Nos. 12 and 13 was high in the r-value, which was 2.02 to 2.20, but low in El, which was 35.8% to 36.8%. Also, these steel samples were somewhat high in σ_(0.2), which was 240 MPa to 250 MPa, supporting a satisfactory dent-resistance. However, since the values of δ and Wca were large, the panel shape of each of these Comparative steel samples was not satisfactory. Further, each of Comparative steel samples Nos. 14 and 15 was low in El, which was 37.0 to 38.5%, and in the r-value, which was 1.51 to 1.69, supporting a poor shapeability.

TABLE 1 Steel (12/93)* Sample No. C Si Mn P S sol.Al N Nb B O (Nb/C) Remarks 1 0.0067 0.02 0.30 0.040 0.008 0.060 0.0022 0.062 tr. 0.0020 1.2 Present invention 2 0.0080 0.06 0.65 0.020 0.012 0.035 0.0030 0.081 tr. 0.0024 1.3 Present invention 3 0.0085 0.14 0.55 0.050 0.01 0.059 0.0020 0.145 tr. 0.0022 2.2 Present invention 4 0.013 0.07 1.20 0.020 0.009 0.070 0.0035 0.141 tr. 0.0017 1.4 Present invention 5 0.010 0.13 0.90 0.055 0.011 0.062 0.0018 0.202 tr. 0.0019 2.6 Present invention 6 0.0072 0.02 0.80 0.025 0.01 0.040 0.0025 0.073 0.0003 0.0025 1.3 Present invention 7 0.011 0.04 0.60 0.040 0.013 0.030 0.0019 0.119 0.0008 0.0020 1.4 Present invention 8 0.0045* 0.05 0.65 0.055 0.01 0.063 0.0025 0.024 tr. 0.0019 0.7* Comparative example 9 0.0081 0.03 0.45 0.064 0.0075 0.055 0.0022 0.050 tr. 0.0025 0.8* Comparative example 10 0.0033* 0.05 0.55 0.035 0.007 0.059 0.0025 0.038* tr. 0.0023 1.5 Comparative example 11 0.019* 0.10 0.75 0.060 0.012 0.070 0.0030 0.191 tr. 0.0022 1.3 Comparative example 12 0.0076 0.06 0.52 0.042 0.009 0.040 0.0025 0.200 tr. 0.0017 3.4* Comparative example 13 0.010 0.05 0.80 0.039 0.01 0.040 0.0024 0.270* tr. 0.0018 3.5* Comparative example 14 0.0070 0.04 0.59 0.015 0.008 0.037 0.0032 0.081 tr. 0.0036* 1.5 Comparative example 15 0.010 0.05 0.80 0.040 0.01 0.038 0.0024 0.100 tr. 0.0043* 1.3 Comparative example *outside scope of present invention

TABLE 2 Steel sample Annealing σ0.2 TS El Average 2% BH ΔYPel σ(ε = 0.02) σ(ε = 0.04) No. method (MPa) (MPa) (%) r-value (MPa) (%) (MPa) (MPa) 1 CA 225 373 42.5 1.88 0 0 283 318 CG 227 370 42.0 1.85 0 0 286 322 2 CA 229 377 43.0 1.95 0 0 289 324 CG 230 375 42.6 1.92 0 0 289 324 3 CA 235 388 45.0 2.20 0 0 294 328 CG 232 390 44.6 2.10 0 0 294 327 4 CA 233 396 42.0 1.97 0 0 293 328 CG 230 392 41.6 1.93 0 0 293 323 5 CA 230 370 42.5 2.15 0 0 288 321 CG 230 375 42.0 2.11 0 0 286 320 6 CA 235 380 42.0 2.00 0 0 298 328 CG 233 376 41.6 1.94 0 0 293 324 7 CA 233 385 43.0 1.99 0 0 295 323 CG 225 380 42.0 1.93 0 0 284 316 8 CA 240 343 41.0 1.70 35 1.0  270*  296* CG 245 345 39.7 1.65 33 0.9  276*  300* 9 CA 260 399 36.5 1.55 40 2.0  290*  315* CG 258 402 35.9 1.51 42 2.2  290*  311* 10 CA 213 358 43.5 2.00 0 0  280*  312* CG 210 357 43.0 1.97 0 0  272*  305* 11 CA 270 410 38.0 1.60 0 0  320*  363* CG 265 404 37.5 1.57 0 0  313*  357* 12 CA 244 386 36.8 2.20 0 0 304 339 CG 240 385 36.0 2.10 0 0 299 335 13 CA 247 400 36.3 2.15 0 0 307 340 CG 250 402 35.8 2.02 0 0 310 345 14 CA 228 370 38.5 1.69 0 0 288 319 CG 225 368 38.2 1.65 0 0 282 319 15 CA 255 406 37.0 1.60 0 0  307*  342* CG 258 404 37.5 1.51 0 0  304*  344* Steel Sample σ(ε = 0.08) 2% PO.1 4% PO.1 8% PO.1 No. (MPa) (N) (N) (N) δ(%) Wca (μm) Remarks 1 385 158 167 183 3(◯) 0.10(◯) Present invention 389 159 168 186 3(◯) 0.10(◯) Present invention 2 387 160 171 186 4(◯) 0.10(◯) Present invention 389 160 171 189 4(◯) 0.15(◯) Present invention 3 397 163 173 192 5(◯) 0.17(◯) Present invention 392 163 173 190 4(◯) 0.14(◯) Present invention 4 392 163 175 191 3(◯) 0.15(◯) Present invention 388 161 170 189 3(◯) 0.14(◯) Present invention 5 390 160 171 190 3(◯) 0.10(◯) Present invention 388 160 169 190 3(◯) 0.13(◯) Present invention 6 395 167 175 193 4(◯) 0.15(◯) Present invention 392 164 173 189 4(◯) 0.12(◯) Present invention 7 390 164 174 188 4(◯) 0.12(◯) Present invention 383 159 168 184 3(◯) 0.09(◯) Present invention 8  355* 166 170 185 7(Δ) 0.26(Δ) Comparative example  361* 165 172 188 8(Δ) 0.30(Δ) Comparative example 9  374* 178 190 193 11(X)  0.50(X) Comparative example  372* 180 187 193 11(X)  0.49(X) Comparative example 10  377* 154 160 172 2(◯) 0.10(◯) Comparative example  373* 148 157 170 2(◯) 0.08(◯) Comparative example 11  419* 182 197 196 12(X)  0.46(X) Comparative example  415* 177 195 190 11(X)  0.44(X) Comparative example 12 408 168 177 189 7(Δ) 0.25(Δ) Comparative example 405 166 173 188 7(Δ) 0.24(Δ) Comparative example 13 416 168 175 191 8(Δ) 0.29(Δ) Comparative example 419 171 181 194 10(Δ)  0.29(Δ) Comparative example 14 388 161 159 190 3(◯) 0.12(◯) Comparative example 382 158 160 187 3(◯) 0.10(◯) Comparative example 15  416* 166 174 191 11(X)  0.27(Δ) Comparative example  415* 165 177 190 11(X)  0.32(Δ) Comparative example *outside scope of formula (1) CA continuous annealing CG continuous galvanizing

Example 2

A molten steel having a composition of steel sample No. 2 of the present invention shown in Table 1 was prepared by melting and casting in a laboratory, followed by casting the molten steel to prepare a slab having a thickness of 50 mm. The slab was treated by a blooming mill to reduce the thickness of the steel sheet to 25 mm, followed by heating the steel sheet at 1250° C. for 1 hour under the atmosphere and subsequently applying a hot rolling treatment to reduce the thickness of the steel sheet to 2.8 mm. The finishing temperature and the coiling temperature in the hot rolling treatment were changed within ranges of 770° C. to 930° C. and 450° C. to 750° C., respectively. Then, the hot rolled steel sheet was pickled, followed by applying a cold rolling to reduce the thickness of the steel sheet to 0.75 mm and subsequently applying a soaking treatment at 825° C. for 90 seconds. Further, a temper rolling was applied at an elongation of 1.2%. The mechanical characteristics and the panel characteristics of the thin steel sheet thus prepared were examined as in Example 1. Table 3 shows the results. The finishing temperature for each of steel samples Nos. 1 to 3 of the present invention was lower than (Ar₃−100)° C. Also, each of these steel samples exhibited a low P0.1 under strains of 2% to 8%, i.e., 139N to 159N, and a high Wca, i.e., 0.35 μm to 0.40 μm, indicating that these steel samples were poor in the dent-resistance and in the shapeability. Further, the r-value for these steel samples was as low as 1.69 to 1.77. The coiling temperature for each of steel samples Nos. 7 and 12 was lower than 500° C. Also, each of these steel samples exhibited a high σ_(0.2) value, i.e., 243 MPa and 248 MPa, respectively, supporting a good dent-resistance. However, the δ value was as high as 8% and the Wca value was as high as 0.30 μm, indicating that these steel samples were poor in the panel shape. The coiling temperature for each of steel samples Nos. 11, 15 and 18 was higher than 700° C. Also, each of these steel samples exhibited a low σ_(0.2) value, i.e., 210 MPa to 216 MPa, and such a low δ value of 2%. However, the Wca value was as high as 0.42 μm to 0.43 μm. Also, the dent-resistance load was low in each of these steel samples. On the other hand, each of steel samples Nos. 4-6, 8-10, 13, 14, 16 and 17, which fell within the scopes specified in the present invention in respect of the finishing temperature and the coiling temperature, was found to be satisfactory in each of the formability, the dent-resistance and the shapeability.

TABLE 3 Steel Finish Coiling sample temperature temperature σ0.2 TS El Average σ(ε = 0.02) No. No. (° C.) (° C.) (MPa) (MPa) (%) r-value (MPa) 1 Steel 2  770** 540 212 375 41.3 1.73  275* 2 600 217 372 42.0 1.69  281* 3 660 215 370 42.0 1.77  280* 4 810 530 230 380 43.0 1.89 289 5 600 227 375 43.5 1.92 285 6 670 225 377 44.0 1.95 285 7 850  470** 243 382 41.0 1.80  293* 8 530 232 377 42.8 1.88 292 9 590 230 370 43.3 1.93 289 10 650 230 373 43.0 1.95 293 11  715** 216 370 43.3 1.82  281* 12 890  450** 248 382 41.2 1.82  301* 13 550 233 371 42.8 1.90 292 14 650 226 378 43.5 1.98 287 15  750** 210 367 42.7 1.83  274* 16 930 550 230 370 42.8 1.91 290 17 650 225 375 43.2 1.95 283 18  750** 212 368 43.7 1.81  277* σ(ε = 0.04) σ(ε = 0.08) 2%, 4%, 8% PO.1 δ Wca No. (MPa) (MPa) (N) (%) (μm) Remarks 1  306*  375* 139-153 2(◯) 0.35(Δ) Comparative example 2  313*  383* 143-159 2(◯) 0.40(Δ) Comparative example 3  311*  380* 144-156 2(◯) 0.40(Δ) Comparative example 4 322 392 152-179 4(◯) 0.15(◯) Present invention 5 317 387 150-175 4(◯) 0.10(◯) Present invention 6 318 388 150-177 3(◯) 0.09(◯) Present invention 7  328*  400* 155-182 8(Δ) 0.30(Δ) Comparative example 8 324 392 154-178 4(◯) 0.10(◯) Present invention 9 320 390 151-178 4(◯) 0.12(◯) Present invention 10 322 392 154-177 4(◯) 0.12(◯) Present invention 11  313*  382* 144-156 2(◯) 0.43(X) Comparative example 12  334*  407* 160-184 8(Δ) 0.30(Δ) Comparative example 13 325 394 155-180 4(◯) 0.07(◯) Present invention 14 321 385 153-171 3(◯) 0.18(◯) Present invention 15  305*  372* 139-152 2(◯) 0.42(X) Comparative example 16 321 390 152-178 4(◯) 0.18(◯) Present invention 17 315 384 150-170 3(◯) 0.17(◯) Present invention 18  309*  377* 142-155 2(◯) 0.42(X) Comparative example *outside scope of formula (1); **outside scope of present invention

Example 3

Molten steel of the composition shown in Table 4 (steel samples Nos. 1 to 15 belonging to Examples of the present invention, with steel samples Nos. 16 to 29 belonging to Comparative Example) were prepared in a laboratory, followed by continuously casting the molten steel to prepare a slab having a thickness of 60 mm. The slab was treated by a blooming mill to reduce the thickness of the steel sheet to 30 mm, followed by heating the steel sheet at 1100° C. for 1 hour under the air atmosphere for the hot rolling process (by roughing mill). After the rough rolling, a finish rolling was applied at 890° C., followed by applying a coiling simulation at 600° C. so as to obtain a hot rolled sheet having a thickness of 3 mm. Then, the hot rolled steel sheet was pickled, followed by applying a cold rolling to reduce the thickness of the steel sheet to 0.75 mm and subsequently applying a continuous annealing at 850° C. for 90 seconds. Alternatively, after the continuous annealing at 850° C. for 90 seconds, a galvanizing was applied at 460° C., followed by applying an alloying treatment at 500° C. Further, 1.0% of temper rolling was applied to the annealed steel sheet or the galvanized steel sheet so as to prepare samples for the experiments. These samples were used for the tensile test (test piece of JIS No. 5; tested in accordance with the method specified in JIS Z 2241) and for measuring 2% BH amount (measured in accordance with the method specified in JIS G 3135), and ΔYPel (restoring amount of yield point elongation of the sample stored at 25° C. for 67 months after the temper rolling). Also, the sample was formed into the model panel shown in FIG. 7 (molded at three levels of the strain of 2, 4 and 8%). After a heat treatment was applied at 170° C. for 20 minutes, the dent-resistance of the panel and the shapeability of the panel were examined. The dent-resistance was evaluated under a load of P0.1, in which 0.1 mm of residual displacement was imparted to the panel (in the following description, expressions of 2%P0.1, 4%P0.1 and 8%P0.1 are used for denoting the panel imparted with molding strain of 2, 4 and 8%, respectively). On the other hand, the panel shapeability was evaluated by the spring back amount δ and the Arithmetic Average Waviness Height Wca (JIS B 0610). The spring back amount δ was defined by using a curvature radius R′ of the formed panel imparted with 2% of strain and a curvature radius R of the press mold, i.e., δ was defined by (R′/R—1)×100. Where δ was not larger than 6%, i.e., δ≦6%, the evaluation was marked by ◯. Where δ was 7 to 10%, i.e., δ=7 to 10%, the evaluation was marked by Δ. Further, where δ was larger than 10%, i.e., δ>10%, the evaluation was marked by x. On the other hand, the surface waviness height each having a length of 25 mm were measured at optional 10 points in the vicinity of the apex of the panel in accordance with the method specified in JIS B 0610, and the average measured value is denoted by Wca. Where Wca was not larger than 0.2 μm, i.e., Wca≦0.2 μm, the evaluation was marked by ◯. Where Wca was larger than 0.2 μm but not larger than 0.4 μm, i.e., 0.2 μm<Wca≦0.4 μm, the evaluation was marked by Δ. Further, where Wca was larger than 0.4 μm and not larger than 0.6 μm, i.e., 0.4 μm<Wca≦0.6 μm, the evaluation was marked by x.

Table 5 shows the results of measurements and evaluations. In samples Nos. 1 to 15 each having a composition falling within the range specified in the present invention, the value of the 2% BH amount was 0 to 26 MPa and the ΔYPel was 0%. Compared with the steel sample of Comparative Example No. 16, in which the amount of C was 0.0025% and the 2% BH amount was 36 to 38 MPa, 2%P0.1, 4%P0.1, 8%P0.1 of the steel samples of the present invention was high, i.e., 150 to 180N, 160 to 192N and 175 to 208N, supporting a high dent-resistance of the panel. Also, since δ≦6% (evaluation of ◯) and Wca<0.2 μm (evaluation of ◯), the steel samples of the present invention were satisfactory in the panel shapeability. Further, concerning ΔYPel, the restoring amount of the yield point elongation was measured for the samples (steel sample No. 6 for the present invention and steel sample 18 for Comparative Example) stored for 18 months at 25° C. after the temper rolling, with the results as shown in FIG. 12. The value of ΔYPel after storage for 18 months for the steel sample No. 6 of the present invention was less than 0.2%, supporting an excellent resistance to natural aging. On the other hand, the value of ΔYPel for the steel sample of Comparative Example 18 was 2.2%, supporting a marked deterioration in the resistance to natural aging.

Steel samples for Comparative Examples 16 to 29, which do not fall within the scope defined in the present invention, exhibited large values of 2%P0.1, 4%P0.1 and 8%P0.1 of 140 to 195N, 151 to 202N and 160 to 213N, respectively, supporting a satisfactory dent-resistance of the panel. However, in steel samples of Comparative Examples Nos. 16, 18, 19, 23, 24 and 29, the 2% BH was 33 to 45 MPa, ΔYPel was not smaller than 0.2%, i.e., ΔYPel≧0.2%, and Wca was larger than 0.2 μm, i.e., Wca>0.2%. In other words, these steel samples of Comparative Examples were inferior to the steel samples of the present invention in the resistance to natural aging and in the panel shapeability. Also, the value of ΔYPel was 0% in each of the steel samples for comparative Examples Nos. 17, 20-22 and 25-28, supporting a satisfactory resistance to natural aging. However, the value of δ for these Comparative Examples was not smaller than 7%, i.e., δ≧7%, indicating that these steel samples were satisfactory in the panel shapeability.

TABLE 4 Steel Chemical component (% by weight) sample sol. No. C Si Mn P S Al N Nb  1 0.0044 0.015 0.31 0.04 0.007 0.06 0.0025 0.04  2 0.0072 0.06 0.67 0.02 0.012 0.035 0.003 0.062  3 0.0088 0.14 0.55 0.05 0.009 0.059 0.0022 0.072  4 0.013 0.08 1 0.015 0.009 0.07 0.0035 0.097  5 0.01 0.17 0.9 0.055 0.011 0.062 0.004 0.077  6 0.0066 0.075 1.2 0.045 0.008 0.042 0.0018 0.046  7 0.011 0.053 0.85 0.033 0.013 0.025 0.0027 0.08  8 0.0059 0.01 0.75 0.06 0.01 0.055 0.0044 0.042  9 0.0071 0.065 0.8 0.045 0.011 0.059 0.0019 0.05 10 0.005 0.035 0.97 0.035 0.0065 0.04 0.0027 tr.  11 0.0095 0.04 0.69 0.05 0.012 0.053 0.0032 tr.  12 0.0066 0.02 1.3 0.039 0.009 0.037 0.002 tr.  13 0.0088 0.1 0.73 0.02 0.01 0.04 0.0025 0.062 14 0.0055 0.062 0.52 0.03 0.008 0.051 0.0024 0.02 15 0.01 0.049 0.33 0.061 0.012 0.069 0.003 tr.  16 0.0025* 0.05 0.65 0.055 0.01 0.063 0.0025 0.01* 17 0.003* 0.05 0.55 0.035 0.007 0.059 0.0025 0.02 18 0.005 0.1 0.75 0.06 0.012 0.07 0.003 0.026 19 0.0085 0.08 1 0.051 0.008 0.037 0.0037 0.05 20 0.01 0.05 0.8 0.039 0.01 0.04 0.0024 0.1 21 0.019* 0.03 0.45 0.064 0.0075 0.055 0.0022 0.15* 22 0.0055 0.07 0.7 0.05 0.01 0.049 0.003 0.027 23 0.011 0.055 0.59 0.04 0.01 0.045 0.002 0.056 24 0.006 0.1 0.73 0.046 0.0085 0.065 0.0032 tr.  25 0.02* 0.065 1.2 0.035 0.011 0.052 0.0025 tr.  26 0.0049 0.1 0.82 0.05 0.007 0.056 0.0024 tr.* 27 0.009 0.045 0.85 0.05 0.01 0.07 0.0029 tr.* 28 0.0055 0.08 0.7 0.05 0.009 0.052 0.002 tr.* 29 0.009 0.04 0.5 0.038 0.01 0.059 0.0026 tr.* Steel Chemical component sample (% by weight) C-(12/93)Nb- No. Ti Zr V B (12/48)Ti* Remarks  1 tr.  tr. tr. tr. −0.0008 Present  2 tr.  tr. tr. tr. −0.0008 invention  3 tr.  tr. tr. tr. −0.0005  4 tr.  tr. tr. tr. 0.0005  5 tr.  tr. tr. tr. 0.0001  6 tr.  tr. tr. tr. 0.0007  7 0.025 tr. tr. tr. 0.0007  8 0.015 tr. tr. tr. 0.0005  9 0.027 tr. tr. tr. −0.0003 10 0.037 tr. tr. tr. 0.0005 11 0.07 tr. tr. tr. −0.0008 12 0.045 tr. tr. tr. 0.0004 13 tr.  tr. tr. 0.0003 0.0008 14 0.032 tr. tr. 0.0013 0.0000 15 0.067 tr. tr. 0.0006 0.0003 16 tr.  tr. tr. tr. 0.0012* Compar- 17 tr.  tr. tr. tr. 0.0004 ative 18 tr.  tr. tr. tr. 0.0016* Example 19 tr.  tr. tr. tr. 0.0020* 20 tr.  tr. tr. tr. −0.0029* 21 tr.  tr. tr. tr. −0.0004 22 0.041 tr. tr. tr. −0.0019* 23 0.03 tr. tr. tr. 0.0017* 24 0.041 tr. tr. tr. 0.0017* 25 0.12* tr. tr. tr. −0.0037 26 tr.* 0.075* tr. tr.  —* 27 tr.* 0.11* tr. tr.  —* 28 tr.* tr. 0.025* tr.  —* 29 tr.* tr. 0.035* tr.  —* Note: mark * represents that the values does not fall within the scope specified in the present invention. Ti* = Ti % − (48/14)N % − (48/32)S % (where Ti* is not larger than 0, Ti* is regarded as 0)

TABLE 5 (Part 1) Steel Sample σ 0.2 TS El 2% BH ΔYPel σ (ε = 0.2) σ (ε = 0.04) σ (ε = 0.08) No. Annealing (MPa) (MPa) (%) (MPa) (%) (MPa) (MPa) (MPa)  1 Continuous 229 370 41 0 0 285 321 388 annealing Continuous 231 368 40 0 0 287 323 391 galvanizing  2 Continuous 230 375 40.5 0 0 290 322 390 annealing Continuous 230 377 39.5 0 0 289 325 388 galvanizing  3 Continuous 237 390 39.3 0 0 296 331 403 annealing Continuous 235 392 38.5 0 0 299 330 400 galvanizing  4 Continuous 230 380 39.5 22 0 290 321 389 annealing Continuous 228 383 38.6 20 0 290 320 385 galvanizing  5 Continuous 237 395 39 2 0 299 332 403 annealing Continuous 238 397 38.3 4 0 297 330 405 galvanizing  6 Continuous 235 395 39.3 24 0 296 328 398 annealing Continuous 237 396 38.1 25 0 299 330 396 galvanizing  7 Continuous 235 385 40.3 23 0 296 329 397 annealing Continuous 237 385 39.1 22 0 300 333 400 galvanizing  8 Continuous 230 392 39 20 0 290 322 389 annealing Continuous 233 393 38 20 0 295 325 392 galvanizing  9 Continuous 235 387 39.5 0 0 296 329 397 annealing Continuous 237 385 38.3 0 0 300 330 400 galvanizing 10 Continuous 232 381 40 18 0 292 323 392 annealing Continuous 235 384 38.5 19 0 295 328 399 galvanizing Steel Sample 2% P0.1 4% P0.1 8% P0.1 δ Wca No. (N) (N) (N) (%) (μm) Remarks  1 150 160 175 4 (◯) 0.1 (◯) Examples 150 162 177 4 (◯) 0.11 (◯) of  2 153 162 177 4 (◯) 0.1 (◯) present 152 163 175 4 (◯) 0.13 (◯) invention  3 156 166 186 5 (◯) 0.1 (◯) 159 165 183 5 (◯) 0.1 (◯)  4 165 177 193 3 (◯) 0.15 (◯) 162 170 188 3 (◯) 0.14 (◯)  5 160 167 188 5 (◯) 0.09 (◯) 160 167 190 5 (◯) 0.13 (◯)  6 176 186 205 4 (◯) 0.15 (◯) 180 188 205 4 (◯) 0.16 (◯)  7 175 186 203 4 (◯) 0.17 (◯) 178 188 205 4 (◯) 0.15 (◯)  8 163 175 190 3 (◯) 0.15 (◯) 170 179 195 4 (◯) 0.15 (◯)  9 156 163 181 4 (◯) 0.08 (◯) 159 165 183 4 (◯) 0.12 (◯) 10 163 173 191 3 (◯) 0.1 (◯) 169 181 200 4 (◯) 0.08 (◯) (Part 2) Steel Sample σ 0.2 TS El 2% BH ΔYPel σ (ε = 0.2) σ (ε = 0.04) σ (ε = 0.08) No. Annealing (MPa) (MPa) (%) (MPa) (%) (MPa) (MPa) (MPa) 11 Continuous 237 395 39.3  0 0 298 331 401 annealing Continuous 236 394 38.4  0 0 296 330 398 galvanizing 12 Continuous 235 387 40 17 0 296 330 397 annealing Continuous 236 389 39.8 18 0 300 330 400 galvanizing 13 Continuous 237 378 40 26 0 297 333 401 annealing Continuous 235 380 39.5 24 0 293 330 391 galvanizing 14 Continuous 233 380 41  0 0 296 325 392 annealing Continuous 235 382 40.6  0 0 294 330 397 galvanizing 15 Continuous 238 398 39 15 0 303 336 405 annealing Continuous 237 395 38.2 16 0 303 331 402 galvanizing 16 Continuous 236 355 43  36*   0.6*  268*  293*  352* annealing Continuous 235 356 42  38*   0.5*  267*  291*  351* galvanizing 17 Continuous 242 368 41.5 15 0  267*  295*  357* annealing Continuous 244 370 40 13 0  273*  310*  366* galvanizing 18 Continuous 245 390 39  37*   0.7*  294*  318*  385* annealing Continuous 245 393 38  39*   0.6*  295*  318*  388* galvanizing 19 Continuous 258 400 38.2  44*  2*  310*  343*  405* annealing Continuous 255 403 37  45*   1.8*  308*  337*  409* galvanizing 20 Continuous 256 408 38  0 0  310*  345*  417* annealing Continuous 260 405 37.2  0 0  315*  344*  413* galvanizing Steel Sample 2% P0.1 4% P0.1 8% P0.1 δ Wca No. (N) (N) (N) (%) (μm) Remarks 11 158 166 185 6 (◯) 0.13 (◯) Examples 156 165 181 5 (◯) 0.1 (◯) of 12 168 181 197 5 (◯) 0.15 (◯) present 173 183 200 5 (◯) 0.14 (◯) invention 13 179 192 208 6 (◯) 0.17 (◯) 172 187 199 5 (◯) 0.18 (◯) 14 156 163 178 4 (◯) 0.12 (◯) 155 165 181 5 (◯) 0.1 (◯) 15 173 185 203 6 (◯) 0.15 (◯) 175 181 200 6 (◯) 0.15 (◯) 16 160 165 177 5 (◯) 0.26 (Δ) Comparative 161 165 179 5 (◯) 0.25 (Δ) Examples 17 140 151 165 8 (Δ) 0.15 (◯) 142 152 160 8 (Δ) 0.16 (◯) 18 183 188 205 9 (Δ) 0.25 (Δ) 185 190 208 10 (Δ) 0.26 (Δ) 19 193 202 210 13 (X) 0.39 (Δ) 195 200 212 12 (X) 0.42 (X) 20 163 179 199 12 (X) 0.19 (◯) 170 178 195 14 (X) 0.25 (Δ) (Part 3) Steel Sample σ 0.2 TS El 2% BH ΔYPel σ (ε = 0.2) σ (ε = 0.04) σ (ε = 0.08) No. Annealing (MPa) (MPa) (%) (MPa) (%) (MPa) (MPa) (MPa) 21 Continuous 268 410 38  0 0  320*  363*  421* annealing Continuous 260 415 37  0 0  310*  345*  409* galvanizing 22 Continuous 256 391 39  0 0  308*  344*  410* annealing Continuous 251 393 40  0 0  303*  332*  400* galvanizing 23 Continuous 261 400 38.4  40*   0.8*  308*  347*  415* annealing Continuous 263 403 37.2  37*   1.2*  310*  343*  420* galvanizing 24 Continuous 257 394 39  40*   0.6*  309*  347*  415* annealing Continuous 262 391 38.2  37*  1*  312*  343*  416* galvanizing 25 Continuous 265 398 38.3  0 0  312*  350*  416* annealing Continuous 268 403 37.1  0 0  319*  315*  422* galvanizing 26 Continuous 258 393 38.7  0 0  308*  340*  407* annealing Continuous 258 390 37 0  0  308*  341*  410* galvanizing 27 Continuous 265 400 38.3 22 0  313*  345*  412* annealing Continuous 268 403 37.1 20 0  320*  350*  420* galvanizing 28 Continuous 237 399 39.3 18 0  310*  345*  419* annealing Continuous 239 400 38 15 0  315*  345*  422* galvanizing 29 Continuous 258 388 38.5  35*   0.4*  304*  347*  403* annealing Continuous 260 391 37.1  33*   0.7*  308*  343*  407* galvanizing Steel Sample 2% P0.1 4% P0.1 8% P0.1 δ Wca No. (N) (N) (N) (%) (μm) Remarks 21 176 193 194 14 (X) 0.42 (X) Comparative 163 180 190 14 (X) 0.4 (Δ) Examples 22 162 178 191 12 (X) 0.25 (Δ) 160 167 183 12 (X) 0.27 (Δ) 23 188 202 212 13 (X) 0.49 (X) 186 199 213 13 (X) 0.44 (X) 24 190 202 212 12 (X) 0.23 (Δ) 188 199 211 13 (X) 0.25 (Δ) 25 163 184 200 13 (X) 0.43 (X) 175 185 205 14 (X) 0.45 (X) 26 162 170 189 11 (X) 0.36 (Δ) 162 173 190 13 (X) 0.35 (Δ) 27 185 195 208 13 (X) 0.4 (Δ) 185 197 208 14 (X) 0.52 (X) 28 181 193 207 7 (Δ) 0.59 (X) 183 192 207 8 (Δ) 0.55 (X) 29 184 200 206 12 (X) 0.44 (X) 185 198 208 13 (X) 0.53 (X) Note: The mark * represents that the values do not fall within the scopes defined in the present invention.

Example 4

Molten steel having compositions of steel samples Nos. 2 and 14 of the present invention shown in Table 4 was prepared by melting and casting in a laboratory, followed by casting the steel to prepare a slab having a thickness of 50 mm. The slab was treated by a blooming mill to reduce the thickness of the steel sheet to 20 mm, followed by heating the steel sheet at 1200° C. for 1 hour under the atmosphere and subsequently applying a hot rolling treatment to reduce the thickness of the steel sheet to 2.8 mm. The finishing temperature and the coiling temperature in the hot rolling treatment were changed within ranges of 750° C. to 930° C. and 440° C. to 750° C., respectively. Then, the hot rolled steel sheet was pickled, followed by applying a cold rolling to reduce the thickness of the steel sheet to 0.75 mm and subsequently applying a continuous annealing (soaking treatment) at 800° C. for 90 seconds. Further, a temper rolling (1.4%) was applied. The thin steel sheet thus prepared was shaped into a model panel shown in FIG. 7 with equivalent strains of 2%, 4% and 8%, followed by applying a heat treatment at 170° C. for 20 minutes, said heat treatment corresponding to the coating-baking treatment. Table 6 shows the results of evaluation of the dent-resistance of the panel (three levels of 2%, 4% and 8% of strains) and of the shapeability of the panel imparted with 2% of strain. Samples Nos. 4-7, 9-12, 15-18, 20, 21, 27-29, 32-34, and 36-39 shown in Table 6 fall within the scope of the present invention. On the other hand, samples Nos. 1-3, 8, 13, 14, 19, 22-26, 30, 31, 35 and 40 represent Comparative Examples.

The finishing temperature for samples Nos. 1-3 and 23-26 for Comparative Examples was lower than (Ar₃−100)° C., which does not fall within the scope defined in the present invention. As a result, these samples for Comparative Examples exhibited a 2% to 8%P0.1 of 140N to 158N and 140N to 165N, and Wca values of 0.38 to 0.43 μm and 0.37 to 0.59 μm, respectively, resulting in failure to obtain a good dent-resistance of the panel and a good shapeability. The coiling temperature for samples Nos. 8, 14, 31, and 35 for Comparative Examples was lower than 500° C. and, thus, each of these samples exhibited a good dent-resistance, i.e., 2 to 8%P0.1 of 160N to 189N. However, the Wca values were 0.23 to 0.45 μm and the δ values were 7 to 8%, indicating a poor panel shapeability.

Further, the coiling temperature for samples Nos. 13, 19, 22, 30, and 40 for Comparative Examples was higher than 700° C. and, thus, each of these samples exhibited an undesirable dent-resistance, i.e., 2 to 8%P0.1 of 145N to 166N. Also, the Wca values were 0.33 to 0.42 μm, indicating a poor panel shapeability.

On the other hand, each of the finishing temperature and the coiling temperature for Nos. 4-7, 9-12, 15-18, 20, 21, 27-29, 32-34, and 36-39 of the present invention fell within the scope defined in the present invention. As a result, 2 to 8%P0.1 was 153 to 188N, supporting a good dent-resistance of the panel. The samples of the present invention were also satisfactory in the δ value, i.e., δ≦5%, and in the Wca value, i.e., Wca<0.2 μm, supporting a good shapeability.

TABLE 6 Steel Finishing Coiling Condition sample temperature temperature σ 0.2 σ (ε = 0.02) σ (ε = 0.04) No. No. (° C.) (° C.) (MPa) (MPa) (MPa)  1 Steel 2   780* 550 215  283*  313*  2 600 218  284*  315*  3 660 217  282*  318*  4 820 530 230 294 325  5 590 230 291 325  6 630 235 294 328  7 680 233 294 326  8 860  460* 245  299*  331*  9 550 231 295 328 10 600 233 294 327 11 640 235 296 330 12 680 230 294 328 13  730* 220  285*  318* 14 900  450* 249  303*  337* 15 540 235 298 330 16 600 232 296 328 17 650 235 299 330 18 680 230 295 330 19  725* 217  283*  316* 20 930 550 235 295 331 21 680 233 295 330 22  750* 220  285*  318* Condition σ (ε = 0.08) 2%, 4%, 8% P0.1 δ Wca No. (MPa) (N) (%) (μm) Remarks  1  379* 140-155 3 (◯) 0.38 (Δ) Comparative example  2  384* 144-155 2 (◯) 0.4 (Δ) Comparative example  3  381* 144-158 5 (◯) 0.43 (X) Comparative example  4 390 155-177 4 (◯) 0.12 (◯) Present invention  5 387 153-175 4 (◯) 0.1 (◯) Present invention  6 394 155-177 4 (◯) 0.18 (◯) Present invention  7 391 155-178 5 (◯) 0.16 (◯) Present invention  8  390* 160-176 7 (Δ) 0.23 (Δ) Comparative example  9 392 158-179 5 (◯) 0.14 (◯) Present invention 10 392 157-179 4 (◯) 0.15 (◯) Present invention 11 394 158-177 5 (◯) 0.08 (◯) Present invention 12 391 156-178 5 (◯) 0.18 (◯) Present invention 13  388* 148-162 3 (◯) 0.36 (Δ) Comparative example 14  397* 161-181 8 (Δ) 0.26 (Δ) Comparative example 15 395 158-178 4 (◯) 0.18 (◯) Present invention 16 390 157-177 4 (◯) 0.12 (◯) Present invention 17 393 159-179 5 (◯) 0.1 (◯) Present invention 18 393 158-179 5 (◯) 0.12 (◯) Present invention 19  385* 146-166 3 (◯) 0.42 (X) Comparative example 20 394 158-178 4 (◯) 0.15 (◯) Present invention 21 394 158-179 5 (◯) 0.19 (◯) Present invention 22  388* 149-165 3 (◯) 0.41 (X) Comparative example Steel Finishing Coiling Condition sample temperature temperature σ 0.2 σ (ε = 0.02) σ (ε = 0.04) No. No. (° C.) (° C.) (MPa) (MPa) (MPa) 23 Steel 14  750*  450* 214  280*  315* 24 550 217  282*  320* 25 650 217  282*  318* 26  750* 215  284*  320* 27 840 550 238 303 335 28 600 235 295 333 29 650 235 297 332 30  730* 220  285*  320* 31 890  440* 247  301*  335* 32 550 235 296 334 33 650 237 297 335 34 680 237 303 335 35 920  460* 250  303*  339* 36 520 236 295 333 37 580 233 297 332 38 640 235 297 335 39 680 231 294 331 40  730* 219  284*  318* Condition σ (ε = 0.08) 2%, 4%, 8% P0.1 δ Wca No. (MPa) (N) (%) (μm) Remarks 23  382* 140-160 3 (◯) 0.4 (Δ) Comparative example 24  382* 145-160 3 (◯) 0.37 (Δ) Comparative example 25  385* 145-165 2 (◯) 0.43 (X) Comparative example 26  385* 147-165 2 (◯) 0.59 (X) Comparative example 27 401 160-185 5 (◯) 0.18 (◯) Present invention 28 400 156-183 5 (◯) 0.15 (◯) Present invention 29 405 157-188 4 (◯) 0.15 (◯) Present invention 30  388* 147-165 3 (◯) 0.33 (Δ) Comparative example 31  405* 160-189 8 (Δ) 0.3 (Δ) Comparative example 32 400 157-183 5 (◯) 0.19 (◯) Present invention 33 401 158-185 5 (◯) 0.18 (◯) Present invention 34 403 160-185 5 (◯) 0.13 (◯) Present invention 35  403* 160-187 7 (Δ) 0.45 (X) Comparative example 36 401 156-185 5 (◯) 0.19 (◯) Present invention 37 403 158-187 5 (◯) 0.19 (◯) Present invention 38 402 157-185 5 (◯) 0.17 (◯) Present invention 39 397 155-181 5 (◯) 0.15 (◯) Present invention 40  385* 145-166 3 (◯) 0.38 (Δ) Comparative example Note: The mark * represents that the values do not fall within the scopes defined in the present invention.

INDUSTRIAL APPLICABILITY

As described above, the present invention makes it possible to manufacture stably a cold-rolled steel sheet and a galvanized steel sheet satisfying the dent-resistance of a panel, the surface shapeability and resistance to natural aging and having a tensile strength of 340 MPa or more, which are required for steels used for an outer panel of a motor car, by specifying the steel composition, the tensile characteristics and the manufacturing conditions. It follows that the present invention is highly valuable in the steel industries and in the motor car industries. 

What is claimed is:
 1. A cold-rolled steel sheet excellent in formability, panel shapeability and dent-resistance, consisting essentially of 0.005 to 0.015% by weight of C, 0.01 to 0.2% by weight of Si, 0.2 to 1.5% by weight of Mn, 0.01 to 0.07% by weight of P, 0.006 to 0.015% by weight of S, 0.01 to 0.08% by weight of sol. Al, not higher than 0.004% by weight of N, not higher than 0.003% by weight of O, 0.04 to 0.23% by weight of Nb, the amounts of Nb and C meeting the relationship given in formula (1), and a balance of Fe and unavoidable impurities, said cold-rolled steel sheet meeting the relationship given in formula (2): 1.0<(Nb %×12)/(C %×93)≦3.0  (1) exp(ε)×(5.29×exp(ε)−4.19)≦σ/σ_(0.2)≦exp(ε)×(5.64×exp(ε)−4.49)  (2) where 0.002<ε≦0.096, ε represents a true strain, σ_(0.2) represents a 0.2% proof stress, and σ represents a true stress relative to ε.
 2. The cold-rolled steel sheet excellent in formability, panel shapeability and dent-resistance according to claim 1, further comprising 0.0001 to 0.002% by weight of B.
 3. A steel sheet coated with a molten zinc excellent in formability, panel shapeability and dent-resistance, which has been obtained by applying a galvanizing to the cold-rolled steel sheet defined in claim 1 or
 2. 4. A method of manufacturing a cold-rolled steel sheet excellent in formability, panel shapeability and dent-resistance consisting essentially of 0.005 to 0.015% by weight of C, 0.01 to 0.2% by weight of Si, 0.2 to 1.5% by weight of Mn, 0.01 to 0.07% by weight of P, 0.006 to 0.015% by weight of S, 0.01 to 0.08% by weight of sol. Al, not higher than 0.004% by weight of N, not higher than 0.003% by weight of O, 0.04 to 0.23% by weight of Nb, the amounts of Nb and C meeting the relationship given in formula (1), and a balance of Fe and unavoidable impurities, said cold-rolled steel sheet meeting the relationship given in formula (2): 1.0<(Nb %×12)/(C %×93)≦3.0  (1) exp(ε)×(5.29×exp(ε)−4.19)≦σ/σ_(0.2)≦exp(ε)×(5.64×exp(ε)−4.49)  (2) where 0.002<ε≦0.096, ε represents a true strain, σ/_(0.2) represents a 0.2% proof stress, and σ represents a true stress relative to ε, said process comprising the steps of: (a) preparing a molten steel and continuously casting said steel; (b) applying a hot-rolling treatment such that a finish rolling is performed at (Ar₃−100)° C. or more to form a hot-rolled steel band and the rolled steel band is coiled at 500 to 700° C.; and (c) continuously applying a cold-rolling treatment and an annealing treatment to the hot-rolled steel band.
 5. A method of manufacturing a cold-rolled steel sheet excellent in formability, panel shapeability and dent-resistance consisting essentially of 0.005% to 0.015% by weight of C, 0.01 to 0.2% by weight of Si, 0.2 to 1.5% by weight of Mn, 0.01 to 0.07% by weight of P, 0.006 to 0.015% by weight of S, 0.01 to 0.08% by weight of sol. Al, 0.0001 to 0.002% by weight of B, not higher than 0.004% by weight of N, not higher than 0.003% by weight of O, 0.04 to 0.23% by weight of Nb, the amounts of Nb and C meeting the relationship given in formula (1), and a balance of Fe and unavoidable impurities, said cold-rolled steel sheet meeting the relationship given in formula (2)  1.0<(Nb %×12)/(C %×93)≦3.0  (1) exp(ε)×(5.29×exp(ε)−4.19)≦σ/σ_(0.2)≦exp(ε)×(5.64×exp(ε)−4.49)  (2) where 0.002<ε≦0.096, ε represents a true strain, σ_(0.2) represents a 0.2% proof stress, and a represents a true stress relative to ε, said process comprising the steps of: (a) preparing a molten steel and continuously casting said steel; (b) applying a hot-rolling treatment such that a finish rolling is performed at (Ar₃−100)° C. or more to form a hot-rolled steel band and the rolled steel band is coiled at 500 to 700° C.; and (c) continuously applying a cold-rolling treatment and an annealing treatment to the hot-rolled steel band.
 6. A method of manufacturing a galvanized steel sheet, said steel sheet being excellent in formability, panel shapeability and dent-resistance, consisting essentially of 0.005 to 0.015% by weight of C, 0.01 to 0.2% by weight of Si, 0.2 to 1.5% by weight of Mn, 0.01 to 0.07% by weight of P, 0.006 to 0.015% by weight of S, 0.01 to 0.08% by weight of sol. Al, not higher than 0.004% by weight of N, not higher than 0.003% by weight of O, 0.04 to 0.23% by weight of Nb, the amounts of Nb and C meeting the relationship given in formula (1), and a balance of Fe and unavoidable impurities, said cold-rolled steel sheet meeting the relationship given in formula (2): 1.0<(Nb %×12)/(C %×93)≦3.0  (1) exp(ε)×(5.29×exp(ε)−4.19)≦σ/σ_(0.2)≦exp(ε)×(5.64×exp(ε)−4.49)  (2) where 0.002<ε≦0.096, ε represents a true strain, σ_(0.2) represents a 0.2% proof stress, and σ represents a true stress relative to ε, said process comprising the steps of: (a) preparing a molten steel and continuously casting said steel; (b) applying a hot-rolling treatment such that a finish rolling is performed at (Ar₃−100)° C. or more to form a hot-rolled steel band and the rolled steel band is coiled at 500 to 700° C.; and (c) continuously applying a cold-rolling treatment and a galvanizing treatment to the hot-rolled steel band.
 7. A method of manufacturing a galvanized steel sheet, said steel sheet being excellent in formability, panel shapeability and dent-resistance, comprising 0.005 to 0.015% by weight of C, 0.01 to 0.2% by weight of Si, 0.2 to 1.5% by weight of Mn, 0.01 to 0.07% by weight of P, 0.006 to 0.015% by weight of S, 0.01 to 0.08% by weight of sol. Al, 0.0001 to 0.002% by weight of B, not higher than 0.004% by weight of N, not higher than 0.003% by weight of O, 0.04 to 0.23% by weight of Nb, the amounts of Nb and C meeting the relationship given in formula (1), and a balance of Fe and unavoidable impurities, said cold-rolled steel sheet meeting the relationship given in formula (2): 1.0≦(Nb %×12)/(C %×93)≦3.0  (1) exp(ε)×(5.29×exp(ε)−4.19)≦σ/σ_(0.2)≦exp(ε)×(5.64×exp(ε)−4.49)  (2) where 0.002<ε≦0.096, ε represents a true strain, σ_(0.2) represents a 0.2% proof stress, and σ represents a true stress relative to ε, said process comprising the steps of: (a) preparing a molten steel and continuously casting said steel; (b) applying a hot-rolling treatment such that a finish rolling is performed at (Ar₃−100)° C. or more to form a hot-rolled steel band and the rolled steel band is coiled at 500 to 700° C.; and (c) continuously applying a cold-rolling treatment and a galvanizing treatment to the hot-rolled steel band. 